The current mainstream of multi-piece golf balls resides in those balls having a cover outer layer of a polyurethane elastomer base material and a cover inner layer of an ionomer resin base material. See JP-A 9-215775, JP-A 11-104271, U.S. Pat. No. 5,885,172 and U.S. Pat. No. 6,213,894.
Such multi-piece golf balls are generally manufactured by furnishing a core of vulcanized polybutadiene base rubber, injection molding or shape molding an ionomer resin thereon to form a cover inner layer of a predetermined gage, and enclosing it with a polyurethane elastomer to form a cover outer layer. Regrettably, difficulties arise in establishing a direct bond between the inner layer ionomer resin and the outer layer polyurethane elastomer. The insufficient joint between layers results in golf balls exhibiting reduced rebound and distance, degraded durability to repeated impact and unstable spin performance.
To overcome the bond problem, a number of proposals were made including blending of a thermoplastic resin in the cover inner or outer layer (JP-A 11-137726), pre-coating of an adhesive or primer on the inner layer (JP-A 10-179795, JP-A 2001-585), and treatment of the inner layer surface with a plasma or corona discharge (U.S. Pat. No. 6,315,915), all for increasing the bond strength between inner and outer layers.
However, all these approaches still left room for improvement. For instance, the blending of a thermoplastic resin and the coating of an adhesive or primer may lead to golf balls with poor rebound properties. The plasma or corona discharge treatment and the coating of an adhesive or primer often achieve insufficient adhesion, with no substantial improvements in physical properties of the ball being ascertained. If the introduction of a new expensive apparatus is necessary for such treatment to be carried out, not only the number of steps, but also the manufacturing cost is increased. It is thus desired to have a manufacturing method capable of improving the adhesion between inner and outer layers in a more simple and effective manner. There is a need for a golf ball in which cover inner and outer layers are tightly bonded.